Electrical systems for converting electrical energy selectively either from DC to AC or from AC to DC are known in the art. Systems which function as inverters, that is, converting electricity from DC to AC, or rectifiers (converting electrical energy from AC to DC) can be found that provide output power in excess of a few hundred kilowatts. These high power systems require multiple converter bridges. When the system is configured as an inverter, each bridge provides, at each three phase output (or "poles"), a square wave signal for each of the output phases of the converter. Typically, each output pole signal is filtered and summed with corresponding phase angle signals from other bridges in an output transformer to provide an output fundamental signal for each output phase of the converter.
The bridge signals are essentially square waves and are therefore characterized by a harmonic signal content which causes the power level of the bridge signals to change each time a pole is switched. The harmonic frequencies of the signal may be determined in a known manner through Fourier series expansion of a square wave signal. As is known, only the odd harmonics are present in the expansion. For square wave fundamental signals measured between output poles of a three-phase system, the third harmonic and multiples thereof (triplens) are inherently cancelled so that, in addition to the fundamental signal, only the odd harmonic signals less triplens will be present. That is, only the first (fundamental), fifth, seventh, eleventh, thirteenth, seventeenth, nineteenth, twenty third, twenty fifth, etc. will be found. It is these harmonic signals which must be cancelled.
Harmonic cancellation circuitry for multi-bridge three-phase converters are known in the art and include the cancellation circuitry disclosed in U.S. Pat. No. 4,204,264. The '264 harmonic cancellation circuitry provides for cancellation of all harmonics of an order less than (6N-1) from the signal presented each of the output phases of three-phase converter. The phased outputs of the bridges provide the associated fundamental signal at a phase angle determined by a phased gate signal generated by a gate signal source.
In general, harmonic signal cancellation is accomplished by adding the output signals of the phase displaced bridges of an inverter in a manner to bring the signal components all to the same phase angle. For three bridges the +20.degree. bridge is retarded 20.degree.. The -20.degree. bridge is advanced 20.degree.. The 0.degree. bridge is not changed in phase angle.
A typical inverter includes a DC signal source configured with a plurality of bridges along with output transformers and phase cancellation circuitry. There are several possible mechanisms which enable the transformers to be connected so that the harmonic signals in the output of the inverter are cancelled. In a known inverter produced for the PC23 Fuel Cell Power Plant by the Toshiba Corporation of Japan, the transformer secondaries are connected in series to cancel harmonics below the seventeenth. Phase shifting is accomplished by "zigzag" connections on two of the three output transformers in order to shift the phase the desired amount.
The aforementioned circuit configuration results in the fundamental signal components adding in phase with harmonic signal cancellation. No output voltage is developed for the fifth, seventh, eleventh and thirteenth harmonics. The seventeenth and nineteenth are the first harmonics present and the pattern repeats every eighteen harmonic numbers. While this configuration uses a minimum of electrical components and power, there are problems with uniform energization of the transformers when the inverter bridges are not operating.